The present invention relates to a sintered, electrical contact material for low voltage power switchgear, comprising AgSnO.sub.2 as well as Bi.sub.2 O.sub.3 and CuO as further metal oxides, and having a total metal oxide content of between 10% and 25% by volume. In addition, the invention relates also to specific methods for the production of such material.
Such a material is the subject of the commonly-owned U.S. patent application Ser. No. 577,750, filed Feb. 7, 1984 entitled "SINTERED, ELECTRICAL MATERIAL FOR LOW VOLTAGE POWER SWITCHING".
Contact materials on the basis of silver and metal oxides (AgMeO) have proven to be particularly advantageous for low voltage power switchgear; e.g., in contactors or automatic circuit breakers. In the past, where cadmium oxide, in particular, has been used as an active component, these contact materials have specifically met the desired electro-technical properties and have proven successful in the long-term use of switchgear. But since, as is known, cadmium is one of the toxic heavy metals and CdO is emitted into the environment as the contact elements burn off, it has been endeavoured for some time to replace the CdO by other metal oxides as completely as possible. The requirements for these materials are that they burn off just as little as do AgCdO materials in the arc; that they have just as little welding force; and, especially, that they heat up just as little when carrying constant current as the proven AgCdO materials for contact elements.
So far, attempts have been made to replace the cadmium by tin or zinc. However, the known contact compositions with AgSnO.sub.2 and AgZnO have been generally unable to reach the high-grade properties of AgCdO contact elements. In particular, when using contact elements made of AgSnO as alternative material for AgCdO, it has turned out that, due to its higher thermal stability, AgSnO.sub.2 develops a higher transfer resistance than AgCdO because an oxide layer forms due to the effects of arcing. In the current-carrying state of the switchgear, this causes impermissibly high temperatures to develop in the contact elements which may damage the switchgear. On the other hand, AgSnO.sub.2 contact elements burn off less than do AgCdO contact elements, resulting in longer life. The required size of the contact elements can therefore advantageously be reduced in comparison to AgCdO, thereby achieving a not inconsiderable saving in silver.
The aforementioned U.S. patent application Ser. No. 577,750 discloses a new sintered contact material for the above purpose, based on AgSnO.sub.2, in which Bi.sub.2 O.sub.3 and CuO as well as, selectively, CdO are added as further metal oxides, and in which the total metal oxide content is between 10% and 25% by volume with the SnO.sub.2 share equal to or greater than 70% by volume of the total oxide quantity. The contact material is produced powdermetallurgically from an internally oxidized alloy powder (so-called "IOAP"). For the cadmium-free alternative, the material of the following composition, in weight percentages, is given as being particularly advantageous:
87.95% Ag; 9.97% SnO.sub.2 ; 0.98 Bi.sub.2 O.sub.3 ; and 1.10% CuO.
Experiments have now shown that the stated material still does not fully meet the requirements of practical application as a contact material.